1. Field of the Invention
The invention relates to wireless communication, and more particularly to a transmit diversity scheme used in a coherent system for estimation of frequency offset in a flat-fading environment.
2. Description of the Related Art
In the past two decades, wireless communication has grown with unprecedented speed from early radio paging, cordless telephone, and cellular telephony to today's personal communication and computing devices. These commercial wireless applications have had a profound impact on today's business world and people's daily lives. It is likely that future breakthroughs in wireless communication will be driven largely by high data rate applications. Increasing the link or channel bandwidth is a simple but costly remedy. It has been shown that Multiple-Input Multiple-Output (MIMO) systems are a viable approach to provide significant capacity improvement over conventional wireless antenna systems. MIMO links may play an important role in next-generation wireless systems.
The system performance of current wireless communication systems is limited by the major channel impairment, that is, signal fading generally caused by multipath propagation. Theoretically, the most effective technique to mitigate multipath fading in a wireless channel is transmitter power control. However, there are two fundamental problems with this approach. The major problem is the required transmitter dynamic range. The second problem is that the transmitter does not have any knowledge of the channel experienced by the receiver. Hence, the channel information has to be fed back from the receiver to the transmitter, which results in throughput degradation and considerable added complexity to both the transmitter and the receiver.
Other effective techniques such as space, time, and frequency diversity can be used to mitigate the effect of fading. Frequency and time diversity can be exploited from time-varying multipath channels while space (or antenna) diversity is achieved using multiple antennas. In the classical space diversity scheme, multiple antennas are often employed to provide diversity at the receiver. Such receiver diversity approaches may not be desirable for mobile handsets because of the cost, size and power considerations. Therefore, transmit diversity schemes have become one of the most interesting topics in wireless communications. U.S. Pat. No. 6,185,258, issued to Alamouti et al., disclose a transmitter diversity technique for wireless communications. The technique provides good diversity gain, but is very sensitive to channel estimation errors. In this patent, the receiver can only be applicable to a path fading, not channel fading, environment since it cannot deal with the carrier frequency offset. In reality, the frequency offset always exists due to the difference of the oscillator frequencies between the transmitter and the receiver or the Doppler frequency shift effect for mobile communications. Tarokh et al. disclose a differential modulation for two transmit antennas and one receive antenna based on Alamouti's method, see “A differential detection scheme for transmit diversity,” IEEE J. Select. Areas Commun., Vol. 18, No. 7 pp. 1169–1194, July, 2000. In this scheme, the receiver executes differential detection and requires no channel state information to accomplish low decoding complexity. Unfortunately, the differential encoding scheme without estimation of frequency offset is suited only to a noncoherent system but not to a coherent system.
In view of the above, what is needed is a novel transmit diversity scheme applicable to a coherent system, unencumbered by the limitations associated with the prior art.